1. Field of the Invention
This invention relates to a control circuit for an insulated-gate semiconductor device such as a power MOSFET and an IGBT used as a switching element in motor driving inverters.
2. Description of the Related Art
FIG. 1 is a diagram showing in simplified form the configuration of a conventional control circuit of a semiconductor device. The control circuit connected to the gate of an IGBT 1 serving as an insulated-gate semiconductor device has a drive circuit 2 that is a series circuit constructed of an npn transistor 3 and a pnp transistor 4. The collectors of both transistors 3 and 4 are connected to a not shown control power supply so that a control voltage V.sub.CC is applied thereto, and the bases thereof connected in parallel to each other are connected to a switching signal source 9 so that an on/off signal 9S is received. Further, the emitters connected in parallel to each other are connected to the gate of the IGBT 1 through a gate series resistor 5. As a result of such configuration, a drive voltage 2S outputted to the side of the emitters in correspondence to the on/off signal 9S controls the IGBT 1 so as to turn on and off.
That is, when the on/off signal 9S from the switching signal source 9 is switched into an on signal, the npn transistor 3 turns on and the pnp transistor 4 turns off, causing the emitter voltage of the npn transistor 3 to increase to the control power supply voltage V.sub.CC. Therefore, the gate voltage of the IGBT 1 is charged in accordance with a time constant determined by the product of the capacitance between the gate and the emitter and the resistance of the gate series resistor 5. When the gate voltage has exceeded the threshold voltage, the IGBT 1 turns on, thereby allowing a main circuit current I.sub.CO to flow between the drain and source of the IGBT 1. Further, when the on/off signal 9S is switched into an off signal, the npn transistor 3 turns off, causing the emitter voltage to drop from the control power supply voltage V.sub.CC to zero. Therefore, the gate voltage of the IGBT 1 is discharged in accordance with the time constant determined by the product of the capacitance between the gate and the emitter and the resistance of the gate series resistor 5. When the gate voltage has dropped below the threshold voltage, the IGBT 1 turns on, thereby blocking the main circuit current I.sub.CO flowing between the drain and source of the IGBT 1. The IGBT 1 remains turned off until an on signal is outputted from the switching signal source 9 again.
In the conventional control circuit, when the on/off signal 9S outputted from the switching signal source 9 is an off signal and when the IGBT 1 is therefore turned off, the gate voltage of the IGBT 1 is gradually increased by the charges stored through not only the capacitance with respect to the collectors but also the stray capacitance at the gate (hereinafter referred to as the "charges stored by gate capacitive coupling"). At this instance, the pnp transistor 4 remains turned off, so that the charges stored at the IGBT 1 by the gate capacitive coupling tend to be discharged to the ground in FIG. 1 through the gate series resistor 5 and the pnp transistor 4. However, being restricted by the fact that the pnp transistor 4 has a higher saturation voltage than the threshold voltage of the IGBT 1 and that the direct current amplifying rate cannot be increased, the stored charges are not sufficiently discharged, which in turn causes the gate potential of the IGBT 1 to gradually increase, and when this potential has exceeded the threshold voltage of the IGBT 1, the IGBT 1 erroneously turns on. This is a problem.
Further, when the discharge current from the gate of the IGBT 1 flows to the gate series resistor 5, a voltage drop occurs at the gate series resistor 5, and the voltage drop causes the gate potential of the IGBT 1 to increase, thereby making it likely to erroneously turn on the IGBT 1. In order to block this potential increase, it is necessary to set the resistance of the gate series resistor 5 to a small value. However, the small resistance leads to increasing the current decreasing speed -dI.sub.CO /dt at the time the IGBT 1 turns off, thereby increasing the turn-off surge voltage caused by such increase in the current decreasing speed. This not only makes it more likely to cause breakage of the IGBT 1 and the erroneous operation of the control circuit, but also causes a vicious circle of increasing the charges stored by gate capacitive coupling. Therefore, the resistance of the gate series resistor 5 cannot be decreased. This is another problem.
On the other hand, when the npn transistor 3 turns on and the pnp transistor 4 turns off by an on signal from the switching signal source 9, the gate potential of the IGBT 1 is increased in accordance with the time constant determined by the product of the resistance of the gate resistor 5 and the capacitance between the gate of the IGBT 1 and the emitter so that the IGBT 1 turns on. The current increasing speed dI.sub.CO /dt at the time the IGBT 1 has turned on is also determined by such time constant. However, if the charges stored by capacitive coupling is introduced into the gate of the IGBT 1, the gate potential increasing speed at the time the IGBT 1 turns on becomes higher than that defined by the time constant, which in turn increases the current increasing rate dI.sub.CO /dt at the time the IGBT 1 turns on. Hence, the forward surge voltage at the time the IGBT 1 turns on is increased and the control circuit is likely to operate erroneously. These are other problems.